The present invention relates generally to methods of and apparatuses for polishing radially expandable endoprothesis adapted to be implanted in a lumen of a tubular organ. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. Endoprostheses correspond to artificial implantable medical devices that are placed inside the body, and stents are examples of these endoprostheses. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumens such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a body passage or orifice. In such treatments, stents reinforce vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty or valvuloplasty) with apparent success.
A treatment involving a stent includes both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a lumen of a tubular organ to a region requiring treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent may be accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into the lumen, advancing the catheter in the lumen to a desired treatment location, expanding the stent at the treatment location, and then removing the catheter from the lumen.
In the case of a balloon-expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing, or crimping, the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath or a sock. When the stent is in a desired body location, the sheath may be withdrawn allowing the stent to self-expand.
Stents have been made of many materials including metals and polymers. Polymer materials include both nonbioerodable and bioerodable plastic materials. In some applications, a polymeric bioerodable stent may be more advantageous than a metal stent due to its biodegradeability and greater flexibility relative to the metal stent. The cylindrical structure of a stent is typically composed of a scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be formed from wires, tubes, or planar films of material rolled into a cylindrical shape. In addition, a medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier. The polymeric carrier can include an active agent or drug. Furthermore, the pattern that makes up the stent allows the stent to be radially expandable and longitudinally flexible. Longitudinal flexibility facilitates delivery of the stent and rigidity functions to hold open a lumen of a tubular organ. Generally, the pattern should be designed to maintain the longitudinal flexibility and rigidity required of the stent. The stent should also have adequate strength in the circumferential direction.
A number of techniques have been suggested for the fabrication of stents from tubes and planar films or sheets. Examples of such techniques include laser cutting or etching a pattern onto a material. Laser cutting may be performed on a planar film of a material which is then rolled into a tubular configuration. Alternatively, a desired pattern may be etched directly onto a tube. Other techniques involve cutting a desired pattern into a sheet or a tube via chemical etching or electrical discharge machining. Laser cutting of stents has been described in a number of publications including U.S. Pat. No. 5,780,807 to Saunders, U.S. Pat. No. 5,922,005 to Richter and U.S. Pat. No. 5,906,759 to Richter.
With respect to polymeric stents, a number of manufacturing methods have been employed. In one method, the polymeric stent may be formed by laser cutting a flat polymeric sheet in the form of rings and links, subsequently rolling the pattern into the shape of the cylindrical stent and then providing a longitudinal weld to form the stent. In another method, a flat polymeric sheet may be chemically etched and then subsequently rolled and welded to form the polymeric stent. Additionally, a polymeric wire may be coiled to form a polymeric stent. In yet another method, a polymeric stent may be formed from a tube by laser cutting a pattern of cylindrical rings and connecting rings in the tube itself. See, e.g., U.S. Pat. No. 6,585,755 to Jackson et al.
In the manufacturing processes previously described, a resultant “rough” stent will be produced. That is, the struts forming the stent will typically have an approximate “square” cross-section with four distinct surfaces as a result of the cutting or etching process. A typical stent is then polished to “round out” the sharp corners in addition to smoothing out the surface of the stent.
Various methods exist for polishing metal stents. Typical polishing methods include electropolishing using an electrolyte solution or laser polishing using a laser, described in more detail in U.S. Pat. No. 5,344,425 to Sawyer, U.S. Pat. No. 6,679,980 to Andreacchi, U.S. Pat. No. 6,375,826 to Wang et al. and U.S. Pat. No. 6,492,615 to Flanagan. The use of abrasives to polish stents is disclosed in U.S. Pat. No. 5,746,691 to Frantzen, U.S. Pat. No. 5,788,558 to Klein and U.S. Pat. No. 6,086,455 to Frantzen.
Polymeric stents may undergo polishing as well. A polymeric stent may be polished using solvents. Solvent polishing smoothes out the surfaces and rounds out “sharp corners” of the struts and connecting elements of a polymeric stent; however, adequately removing the solvent and minimizing the residual solvent are problems associated with this method. Accordingly, methods are needed for polishing polymeric stents.